1. Field of the Invention
The present invention relates to a method, an apparatus, and a system for processing information by attaching information to digital data (such as data representing a document, an image, or music) to protect a copyright of the digital data, to protect the digital data from being counterfeited, and to manage the digital data, and also relates to a storage medium storing a computer-readable program to carry out the method.
2. Description of the Related Art
Due to rapid advances in computers and computer networks, a diversity of digital data, such as document data, image data, and audio data, is handled by computers and computer networks. Since such digital data is easily copied, copyright of the digital data is frequently violated. Copyright information and user information are occasionally embedded using a digital watermark technique, without drawing attention to them, in a manner such that those items of information are not recognized either in vision or by hearing. In this way, a user of the digital data can extract the copyright information from the digital data, and can examine the copyright information to determine whether the digital data has been distributed by an unauthorized third party. A copyright holder of the digital data can extract the user information from the digital data, and can examine the user information to determine whether the digital data has been illegally obtained by the user.
A first requirement of the digital watermark technique is that the digital watermark embedded in the digital data go unnoticed by the senses of human beings. In other words, a digital watermark must be embedded without degrading the quality of the original digital data (quality).
A second requirement of the digital watermark is that the embedded digital watermark remain embedded. Even if the digital data is edited, for example, compressed or filter processed, the embedded digital watermark must not be lost (robustness).
A third requirement of the digital watermark is that the amount of information of the digital watermark to be embedded into the digital data be selected depending on the application of the digital data (the amount of information).
The three requirements imposed on the digital watermark are mutually traded-off with respect to each other.
For example, a scheme of digital watermarking that embeds information with a sufficient robustness suffers from a relatively large quality degradation, and the amount of information embedded becomes small.
Schemes of digital watermarking that embed the digital watermark in multi-level digital data representing still image are divided into two major types. In one type, a digital watermark is embedded in a spatial area, and in the other type, a digital watermark is embedded in a frequency domain.
The scheme of embedding the digital watermark into a spatial area of the digital data is not so much degraded in quality, but is less robust. The scheme of embedding the digital watermark in the frequency domain of the digital data suffers from a significant quality degradation, but is more robust. Among the various schemes of embedding the digital watermark in a spatial area of the digital data and the schemes of embedding the digital watermark in the frequency domain of the digital data, some are robust but the amount of information embeddable is small, and others suffer less from quality degradation but are less robust.
It is difficult for a copyright holder of digital data to select an appropriate embedding scheme for a digital watermark in the digital data from among numerous schemes.
The scheme used in embedding the digital watermark is related to a scheme of extracting the digital watermark in one-to-one correspondence, and is generally incompatible with another scheme.
If an individual is not aware of the type of scheme through which information is attached to the digital data, the information cannot be extracted from the digital data.